Pressure-assisted molding and carbonation of cementitious materials

ABSTRACT

A method is disclosed for rapidly carbonating large cement structures, by forming and hardening cement in a mold under high carbon dioxide density, such as supercritical or near-supercritical conditions. The method is more reliable, efficient, and effective than are post-molding treatments with high-pressure CO 2 . Cements molded in the presence of high-pressure CO 2  are significantly denser than otherwise comparable cements having no CO 2  treatment, and are also significantly denser than otherwise comparable cements treated with CO 2  after hardening. Bulk carbonation of cementitious materials produces several beneficial effects, including reducing permeability of the cement, increasing its compressive strength, and reducing its pH. These effects are produced rapidly, and extend throughout the bulk of the cement—they are not limited to a surface layer, as are prior methods of post-hardening CO 2  treatment. The method may be used with any cement or concrete composition, including those made with waste products such as fly ash or cement slag. Surface carbonation is almost instantaneous, and bulk carbonation deep into a form is rapid. By combining molding, curing, and carbonation into a single step, carbon dioxide is better distributed throughout the entire specimen or form, producing a uniform product.

This is a continuation of co-pending application Ser. No. 09/170,480, filed Oct. 13, 1998, now allowed with the issue fee paid; which claims the benefit of the Oct. 15, 1997 filing date of provisional application Ser. No. 60/109,799 under 35 U.S.C. § 119(e); the entire disclosures of both of which prior applications are hereby incorporated by reference.

This invention pertains to carbonation of cementitious materials, particularly to carbonation of cements using supercritical or high density carbon dioxide.

Above a compound's “critical point,” a critical pressure and temperature characteristic of that compound, the familiar transition between gas and liquid disappears, and the compound is said to be a “supercritical fluid.” Supercritical fluids (SCFs) have properties of both gasses and liquids, in addition to unique supercritical properties. A supercritical fluid is compressible like a gas, but typically has a density more like that of a liquid. Supercritical fluids have been used, for example, as solvents and as reaction media. The critical pressure and temperature for carbon dioxide are 1071 psi and 31.3° C. The viscosity and molecular diffusivity of a supercritical fluid are typically intermediate between the corresponding values for the liquid and the gas. Compounds below, but near, the critical temperature and pressure are sometimes termed “near-critical.”

Hardened or cured cements have sometimes been reacted with high pressure or supercritical CO₂ to improve their properties. Supercritical and near-critical CO₂ increase the mobility of water that is already present in the cement matrix, water bound as hydrates and adsorbed on pore walls. A pore in the cement may initially contain supercritical or near-critical CO₂ at the pore entrance, a dispersed water phase associated with the pore walls, and possibly free water at the CO₂/water interface. The high CO₂ pressure increases the solubility of CO₂ in the dispersed aqueous phase. A concentration gradient of CO₂ is thus produced in the concrete pores. Carbon dioxide may then react with various cement components, particularly hydroxides of calcium. (As used in the specification and claims, the term “hydroxides of calcium” includes not only Ca(OH)₂, but also other calcareous hydrated cement components, e.g., calcium silicate hydrate.)

Densification Reactions

Carbonation reduces the permeability of cement, typically by 3 to 6 orders of magnitude. This reduction in permeability has been attributed to precipitation of carbonates in the micropores and macropores of the cement. For example, in cement grout carbonation shifts a bimodal pore distribution (pores around 2-10 nm in diameter and pores around 10-900 nm) to a unimodal distribution (pores around 2-10 nm in diameter only). Reduced permeability and smaller pore diameters slow rates of diffusion in carbonated cements. For example, Cl⁻ and I⁻ diffusion coefficients have been reported to be 2 to 3 orders of magnitude lower in carbonated cement than in noncarbonated cement, as have carbon-14 migration rates. (Lower Cl⁻ and I⁻ diffusion rates indicate greater resistance to salt intrusion. Salt intrusion is undesirable, as it can lead to fracturing or cracking.) Curing cement grout with carbon dioxide increases the strength and dimensional stability of a cement. The pH of cement in fully carbonated zones is lowered from a basic ˜13 to a more neutral value of ˜8, allowing the reinforcement of the cement with polymer fibers such as certain polyamides (e.g., nylons) that are incompatible with normal cements.

Carbonation of cement is a complex process. All calcium-bearing phases are susceptible to carbonation. For calcium hydroxide (portlandite) the reaction is

Ca(OH)₂+CO₂→CaCO₃+H₂O

The calcium carbonate may crystallize in one of several forms, including calcite, aragonite and vaterite. Calcite is the most stable and common form.

In this reaction, calcium hydroxide (Ca(OH)₂) is assumed first to dissolve in water, after which it reacts with CO₂. Following reaction, the calcium carbonate (CaCO₃) precipitates. Atmospheric concentrations of CO₂ (˜0.04%), do not react appreciably with completely dry concrete. Conversely, if the concrete pores are filled with water, carbonation at low pressure essentially stops before bulk carbonation of a thick cement form can occur, because the solubility and diffusivity of CO₂ in water are low under such conditions. However, bulk carbonation of cement can occur at atmospheric pressure and ambient temperatures after years of exposure to atmospheric carbon dioxide.

High pressure conditions have previously been used to carbonate the surface layers of hardened cements. However, problems resulting from bulk carbonation of hardened cements have been reported. For example, the volume changes associated with conversion of calcium hydroxide to calcium carbonate have been reported to cause microcracking and shrinkage, at least under certain conditions.

Supercritical Fluids in Cementitious Materials

Supercritical and near-critical fluids confined in narrow pores have properties that are often quite different from those of a bulk gas. Because supercritical fluids are highly compressible, a surface or wall potential can produce a strong, temperature-dependent preferential adsorption, which might not occur at all at lower fluid densities. For example, a water layer on the solid surfaces is believed to be necessary to initiate carbonation reactions. Water is, in turn, a product of carbonation. At lower pressures water can completely fill the pores and thereby limit or even prevent carbonation; in such cases the sample must be dried for carbonation to resume. However, saturation and supersaturation of water in a CO₂-rich phase is possible at high pressure, because phase separation in the concrete pores is slower than the carbonation reaction. Also, at high pressures carbon dioxide may adsorb onto the solid surfaces, along with water. The pore environment may eventually consist of a fluid phase of water and dissolved CO₂, with mostly water but some CO₂, adsorbed onto the walls of the concrete pores. At high pressures solubility of CO₂ in water increases.

E. Reardon et al., “High Pressure Carbonation of Cementitious Grout,” Cement and Concrete Research, vol. 19, pp. 385-399 (1989) discloses treating a solid, hardened, cementitious grout with carbon dioxide gas at pressures up to 800 psi, and notes that this process can sometimes cause physical damage to specimens, including fracturing due to dehydration and shrinkage.

J. Bukowski et al., “Reactivity and Strength Development of CO₂ Activated Non-Hydraulic Calcium Silicates, Cement and Concrete Research, vol. 9, pp. 57-68 (1979) discloses treating non-hydraulic calcium silicates with CO₂ up to 815 psi, and notes that both the extent of the carbonation reaction and the compressive strength of the carbonated materials increased with treatment pressure.

U.S. Pat. No. 4,117,060 discloses a method for the manufacture of concrete, in which a mixture of a cement, an aggregate, a polymer, and water were compressed in a mold, and exposed to carbon dioxide gas in the mold prior to compression, so that the carbon dioxide reacts with the other ingredients to provide a hardened product.

U.S. Pat. No. 4,427,610 discloses a molding process for cementitious materials, wherein the molded but uncured object is conveyed to a curing chamber and exposed to ultracold CO₂.

U.S. Pat. No. 5,518,540 discloses treating a cured cement with dense-phase gaseous or supercritical carbon dioxide. The patent also mentions using supercritical carbon dioxide as a solvent to infuse certain materials into a hardened cement paste. See also U.S. Pat. No. 5,650,562.

U.S. Pat. No. 5,051,217 discloses a continuous stamping and pressing process for curing and carbonating cementitious materials. CO₂ was admitted at low pressures, and could later be compressed to higher pressures in one segment of the apparatus, a segment through which an afterhardening cement mixture passed continuously. The apparatus was said to be quasi-gas-tight. Only a portion of the uncured form was subjected to high pressure at any given time. The ratio of the mass of CO₂ to the mass of the uncured cement was relatively low, apparently always under 0.002 (extrapolating from data given in the specification).

F. Knopf et al., “Densification and pH Reduction in Cement Mixtures Using Supercritical CO₂,” Abstract of paper to be presented at 1997 annual meeting of the American Institute of Chemical Engineers, available on the Internet in July 1997 at

http://www1.che.ufl.edu/meeting/1997/annual/session/100//h/index.html

discloses some of the inventors' own work, work that is disclosed in greater detail in the present specification.

We have discovered that a superior method to rapidly carbonate large cement forms or structures is to shape and harden the cement in a mold under high carbon dioxide pressure, at supercritical, near-supercritical, or high CO₂ density conditions. In other words, contrary to previous teachings, supercritical, near-supercritical, or high density CO₂ is reacted with cement while the cement is still in an uncured state. The novel carbonation method is more reliable, efficient, and effective than are post-molding treatments with high-pressure CO₂, or treatments using low temperature, low pressure CO₂. The novel method is more effective and reliable than methods that admit relatively small amounts of CO₂ to a mold at relatively low pressure, and then compress the uncured mixture. The novel method is more effective in penetrating voids with CO₂, and is therefore more efficient in converting hydroxides of calcium to CaCO₃. Cements molded in the presence of high-pressure CO₂ are significantly denser than otherwise comparable cements having no CO₂ treatment, and are also significantly denser than otherwise comparable cements treated with CO₂ after hardening.

The novel bulk carbonation of cementitious materials produces several beneficial effects, including reducing permeability of the cement, increasing its compressive strength, and reducing its pH. These effects are produced rapidly, and extend throughout the bulk of the cement—they are not limited to a surface layer, as are prior methods of post-hardening CO₂ treatment. The novel method may be used with any cement or concrete composition, including those made with waste products such as fly ash or cement slag. Surface carbonation is almost instantaneous, and bulk carbonation is rapid even with forms several centimeters thick, tens of centimeters thick, or thicker. By combining molding, curing, and carbonation into a single step, carbon dioxide is better distributed throughout the entire specimen or form, producing a uniform carbonated cement product. In particular, it is believed that this is the first cured cement in which all interior portions of the cement that are at least 1 mm from the nearest surface of the cement comprise interlocking calcium carbonate crystals that are at least 10 μm in diameter.

Bulk carbonation of cement with supercritical CO₂ in our laboratory has produced a dense layer of interlocking calcium carbonate (calcite) crystals in minutes. The crystals are an order of magnitude larger in diameter (˜10 μm) than has been previously reported for calcite crystals in the interior of cements. The novel process produces concretes with improved durability and higher compressive strengths.

Uses for concretes based on the novel, bulk-carbonated cements are numerous. The higher compressive strength allows the use of thinner blocks and less material for a given strength requirement. For example, the stronger concrete may be used to make lighter weight, fire-resistant structural panels or roofing tiles. Cement roofing is rapidly gaining acceptance. These roofs last essentially for the lifetime of the home, have a Class A fire rating, and can be cast into any desired appearance. Costs should be competitive with those for shorter-lived asphalt roofing materials.

Low-cost reinforcing fibers may be used in bulk carbonated cements due to the near-neutral pH of these materials. Many potential reinforcing fibers are incompatible with the higher pH found in most cements, e.g. the pH ˜13 of conventional Portland cements. For example, it has been estimated that 3-4 billion pounds of carpet fiber per year are land-filled in the United States. Recycled carpet polymers could instead be used to reinforce these cement structures of near-neutral pH, transforming old carpets from a waste product into a useful resource.

Carbonated cementitious materials can also be used for building artificial reefs. Near-neutral pH's are necessary for the growth of most marine organisms.

Carbonation and polymer reinforcement produce concretes with greater resistance to chemical attack, a property that is useful, for example, in the petroleum, mining, metallurgical, and chemical industries. Bulk-carbonated cements have essentially no die-swell or warpage, an advantage in the ceramics industry.

Preparation of Carbonated and Molded Samples

Comparison samples using previously cured cements were prepared in an existing SCF continuous treatment system. Liquid CO₂ was compressed by a positive displacement diaphragm compressor (American Lewa model ELM-1) to 1500 psi. The compressed CO₂ was stored in surge tanks to dampen pressure fluctuations. The pressure was controlled by a Tescom regulator (model 44-1124) to within ±5 psi. Pressure was monitored by a Heise digital pressure gauge (model 710A). The specimen (10 mm by 10 mm by 40 mm) was held in a tube immersed in a Plexiglas 25° C. constant temperature bath. The CO₂ flow rate was ˜0.8 g/s, and the run time was 1 hour.

A prototype device was constructed to evaluate the novel one-step method for molding, curing, and supercritical (or near-critical or high density) CO₂ treatment. Specimens were treated in a simple cylindrical mold operated by a piston, which was sealed on its outer surface by O-rings. CO₂ gas (at ˜700 psi) was introduced below the piston. The pressure above the piston was rapidly increased using water as a driver fluid. The increased pressure initiated the molding process. As the piston moved rapidly toward the sample, the gas pressure above the sample rose to equalize. But simultaneously the CO₂ reacted with the cement, tending to lower the pressure. A 2000 psi water pressure was applied to the piston, and the samples were generally molded for ˜3 hours, although shorter or longer times can be used. The molded specimens in the prototype embodiment were cylindrical, 39 mm diameter by 13 mm height. The prototype unit allowed various modes of CO₂ addition to be studied, without the complexities inherent in filling the mold with uncured cements under pressure. However, the scope of the invention is not limited by the manner used to fill the mold. The amount of CO₂ added to the cement matrix could be readily controlled by adjusting the initial height of the piston above the cement.

Characterization of Chemical and Physical Properties of Cements

The porosities of conventionally cast samples (i.e., conventionally molded without high pressure CO₂) and samples produced by the novel process were determined indirectly by measuring surface areas at a fixed initial composition. Higher surface areas are often associated with void-filling and therefore with decreased pore volumes, when small pores are created from larger pores without significant pore closure. The amount of nitrogen or other inert gas adsorbed (in determining surface area) includes contributions from capillary condensation in small pores. However, as voids are completely filled surface areas decrease significantly. A discussion of physical adsorption mechanisms in porous materials can be found in standard works on this subject, for example, D. M. Ruthven, Principles of Adsorption and Adsorption Processes (1984).

Thus an increase in surface area upon carbonation indicates a small reduction in voidage, while a decrease in surface area indicates almost complete closure of voids in the specimen, accompanied by densification. Surface areas were estimated using the one-point BET method at 30% relative saturation, using a Micromeritics 2700 Pulse Chemisorption apparatus. Water was first removed under vacuum at 1 torr for 24 h at ambient temperature, then under flowing N₂/He for at least 2 h. The surface areas of selected samples were checked by the full BET N₂ adsorption method using an Omnitherm (model Omnisorp 360) adsorption apparatus. The pore volume was determined in water by displacement (Archimedes' principle). All specimens used in density and porosity measurements were dried under vacuum at 1 torr at ambient temperature prior to measurement.

A Scintag PAD-V automated X-ray Powder Diffractometer was used to identify crystalline phases. Specimens were step-scanned from 3-60° 2θ, at a 0.02° step size, 3 second/step. A Perkin-Elmer thermogravimetric analyzer was used to quantify weight losses from water evolution (from hydrates), hydroxide (e.g., Ca(OH)₂) to oxide (e.g., CaO) conversions, and carbonate (e.g., CaCO₃) to oxide (e.g., CaO) conversions. The carrier gas was helium at 1 atm. The temperature program was 200-700° C., 5° C./min, hold at 700° C.

Results, Post-Treated Samples

The “post-treatments” (i.e., carbonations of previously cast samples) used near-critical CO₂ (1500 psi and 25° C.). The CO₂ density at these conditions was 0.83 g/cm³, well above the density at the critical pressure and temperature (0.46 g/cm³). Table 1 summarizes X-ray diffraction (XRD) results for five different concrete mixes. The samples for the XRD measurements were taken from the surfaces of the specimens. For each mix both a control sample (no carbonation) and a test sample (carbonated) were measured. The reported weights of the additives were normalized to the initial weight of concrete. For all samples, a weight ratio of 0.603 water to 1.0 cement (ASTM Type III) was used in the initial mix. The five mixes represent typical fast set concretes, some of which included one or more of the following additives: glass fibers, Kevlar fibers, calcite, lime, and a plasticizer.

TABLE 1 XRD Phase Characterization of Carbonated Specimens, Continuous Flow Treatment Ratio of XRD peak heights, portlandite/calcite Additives Control 1 2.9 none Test 1 0.029 Control 2 3.9 0.021 lime Test 2 <0.035 0.007 calcite Control 3 4.1 0.021 calcite Test 3 0.08 0.022 WRDA 19 plasticizer Control 4 2.6 0.105 lime Test 4 0.09 0.021 calcite Control 5 3.6 0.105 lime 0.021 calcite 0.022 WRDA 19 plasticizer Test 5 <0.035 0.007 E-glass fiber 0.010 Kevlar 49 fiber

The portlandite peak reported in Table 1 occurred at 18.1° 2θ, and the calcite peak at 29.5°. The reported ratios of portlandite to calcite are not strictly quantitative, because detailed calibrations of peak height versus the weight of a given phase were not made, and also because careful microtome sectioning procedures were not used. Nevertheless, the five control samples showed a reasonably consistent ratio range, 2.6-4.1.

As compared to the controls, the test samples showed a significant increase in calcite (CaCO₃) peak heights, and a corresponding decrease in portlandite (Ca(OH)₂) peak heights. The relative ratio of P/C (portlandite/calcite) for the control and test samples (i.e., (P/C)_(control)/(P/C)_(test)) ranged from a low of 29 for sample 4 to a high of 111 for sample 2. Despite the semi-quantitative nature of these initial XRD measurements, it is still clear that carbonation caused a 1-2 order of magnitude change in the ratio of portlandite to calcite in samples taken from the surface. These experiments show that the presence of typical cement additives did not hinder the carbonation process substantially.

Scanning electron microscope (SEM) photomicrographs showed qualitatively similar appearances for control and test samples at magnification 33×: individual, rounded sand grains coated with the cement. At higher magnifications, 650× and 3700×, significant differences in the crystalline structures became apparent. Before carbonation, the cement comprised primarily calcium silicate hydrate, calcium hydroxide, and ettringite. The carbonated cement, by contrast, showed large calcium carbonate crystals (average diameter 10 μm), with partially developed crystal faces. The average grain size was an order of magnitude greater than that previously reported for carbonated cements. The calcium carbonate crystals formed interlocking grains, suggesting that permeability of the cement was thereby reduced. Also, adhesion between the carbonated layer and the noncarbonated layer, as well as adhesion between the carbonated layer and aggregate, both appeared to be good.

Derivative thermogravimetric analysis (TGA) of a Portland cement mortar before and after carbonation was used to estimate content of calcium carbonate and hydroxides of calcium. The complex chemical nature of a typical cement precludes exact quantitation by TGA, so the TGA results are considered to provide relative comparisons only. A large increase in calcium carbonate content following carbonation was evident, as was a proportional decrease in the content of hydroxides of calcium. The content of ettringite and other stable hydrates appeared to be unaffected by the carbonation.

The SEM micrographs suggested that surface carbonation was extensive. Derivative thermogravimetry, on the other hand, indicated that about half of the hydroxides of calcium did not undergo any change. This discrepancy is explained by the fact that the SEM probed only the top few micrometers of the surface, while the thermal analysis was representative of the top several millimeters of the sample. Thus the deeper one probed into the sample, the lower the degree of carbonation for the post-treated samples. As shown below, the results were quite different for samples produced by the novel supercritical molding treatment.

Results, Molded Specimens; and Comparisons to Post-Treated Specimens

The details of the treatments and initial compositions used in the molding experiments are given in Tables 2 and 3. All initial cure times were 3 hours. All comparison samples were prepared in the molding device with 2000 psi water pressure on the driver side. Some comparison samples were set with air only (i.e., with no more than ambient levels of CO₂.) Some comparison samples were set in air for three hours initially, and the partially cured materials were then contacted with CO₂ for an additional two hours.

TABLE 2 Composition and Treatments for Molded Portland Cement (PC) and Fly Ash Samples Mass of Mass of 5 M NaOH Fiber Type and Mass, PC or Solution, as a as a Percentage of Sample Number Fly Ash Percentage of Mass Mass of PC or and Description (g) of PC or Fly Ash Fly Ash 3A-PC, set in air 50 32 polypropylene, 1.4 3B-PC, set with 50 32 polypropylene, 1.4 CO₂ 2A-fly ash, set 25 40 none in air 2B-fly ash, set 25 40 none with CO₂ 11-fly ash, set 25 40 none with water P = 2000 psi, then CO₂ 5-fly ash, set 25 45 polypropylene, 1.6 with CO₂ 15-fly ash, set in 25 40 polypropylene, 1.6 air then CO₂ 16-fly ash, set 25 44 nylon, 1.6 with CO₂, foamed¹ 17-fly ash, set 25 44 polypropylene, 1.6 with CO₂, foamed¹ ¹foamed with aqueous solution comprising 73% 5 M NaOH and 27% aqueous (30 wt %) H₂O₂

TABLE 3 Composition and Treatments for Molded Cement Slag Samples Mass of Mass of 5 N NaOH Fiber Type and Cement Solution, as a Mass as a Sample Number Slag Percentage of Mass Percentage of and Description (g) of Slag Mass of Slag 4-set with CO₂ 25 44 polypropylene, 1.4 6-set in air, foamed¹ 25 44 0 8-set in air 25 40 polypropylene, 1.6 9-set with CO₂ 30 43 polypropylene, 4.3 10-set with CO₂, 25 45 0 foamed² 12-set in air, CO₂ 25 45 0 post-setting ¹foamed with aqueous solution comprising 55% 5 M NaOH and 45% aqueous (30 wt %) H₂O₂ ²foamed with aqueous solution comprising 76% 5 M NaOH and 24% aqueous (30 wt %) H₂O₂

For the fly ash and cement slag specimens, a 5 M NaOH solution was used to reduce curing times, following the method of U.S. Pat. No. 5,435,843. In some experiments, H₂O₂ was used as a foaming agent to see whether it would affect contact between the CO₂ and the cements. The Portland cement used was Type I. The fly ash was Class C. The cement slag was standard pig-iron blast furnace slag.

After demolding, sectioned samples were tested for increases in carbonate content by TGA. The reactions used to estimate Ca(OH)₂ and CaCO₃ content were as follows:

Hydrates→Silicates , Carbonates (T<300° C.)

MgCO₃→MgO+CO₂ [MW=44] (T ˜300-350° C.)

Ca(OH)₂ [MW=74.1]→CaO+H₂O [MW=18] (350° C.<T<450° C.)

CaCO₃ [MW=100.1]→CaO+CO₂ [MW=44] (T>600° C.)

Hydroxylated Silicas, Aluminas→SiO₂, Al₂O₃+H₂O (T<650° C.)

Other Carbonates→Oxides+CO₂ (T>500° C.)

In most instances the MgCO₃ peak could not be resolved from the Ca(OH)₂ peak. Also, the final dehydrations of the surfaces of other hydroxides such as SiO₂ take place at temperatures that overlap CaCO₃ decomposition. The TGA results should therefore be viewed as estimates of the amounts of Ca(OH)₂ and CaCO₃ in these materials. The TGA results are nevertheless useful in relative comparisons of carbonated versus non-carbonated (but otherwise identical) materials.

Standard samples were used to calibrate appropriate temperature ranges for the dehydration and decarbonation reactions in the TGA analysis. Each standard was a homogeneous physical mixture, containing {fraction (2/3+L )} mold specimen 3A (Portland cement, set in air), and {fraction (1/3+L )} of the additive being tested. These components were ground to a powder with a mortar and pestle. The additives used in separate samples were as follows: CaCO₃, which produced a high-temperature reaction; Ca(OH)₂, which produced a range of multiple dehydrations from ˜350-450° C.; Al(OH)₃, for which bulk dehydration occurred at low temperatures, in the hydrate-loss region; and Na₂SiO₃, which produced a peak at ˜570-640° C., an evolution of water from silicate surfaces that can affect quantitation of the carbonate peak—however, the relatively small size of this peak suggests that rough quantitation of CaCO₃ by TGA is still possible. Tables 4 and 5 give the TGA results for the molded samples. In the Tables, the designations “M” and “T” refer to samples that were removed from the middle of the specimen and the top surface of the specimen, respectively.

TABLE 4 TGA Results, Fly Ash Samples % Water Loss % Hydroxide as % Carbonate as Sample from Hydrates Ca(OH)₂ CaCO₃ 2A-fly ash, set in 2.8 9.6 6.5 air 2B-fly ash, set with 1.6 3.3 13.7 CO₂ 11T 2.3 6.0 5.9 11M 2.7 5.2 6.0 5M 0.89 3.8 10.7 5T 0.74 4.0 11.0 15-fly ash, set in 0.68 13.3 15.1 air, CO₂ post-setting 16-fly ash, set with 0.55 3.7 14.5 CO₂, foamed

TABLE 5 TGA Results, Cement and Cement Slag Samples % Water Loss % Hydroxide as % Carbonate as Sample from Hydrates Ca(OH)₂ CaCO₃ 3A, set in air 1.5 16.0 5.1 3B, set with CO₂ 1.3 13.4 7.0 4, slag, set with 0.86 18.5 7.7 CO₂ 6, slag, set in air, 0.45 14.4 1.1 foamed 8, slag, set in air 1.8 14.7 3.8 9-slag, set with 1.1 16.7 5.8 CO₂ 10-slag, set with 0.56 12.6 2.3 CO₂ foamed 12-slag, set in air, 1.2 12.4 1.6 CO₂ post-setting

Note in Table 4 a general increase in measured CaCO₃ content for all the CO₂-molded samples as compared to the non-carbonated samples. When CO₂ was not used directly in the molding process, but was instead applied as a post-cure treatment in the mold, the measured carbonate content sometimes increased (sample 15), and sometimes did not (sample 11). In Table 5 the carbonate content increased where CO₂ was used in the molding, except for one of the H₂O₂-foamed samples. However, the carbonate content did not increase when CO₂ was used to treat an already-hardened cement slag (sample 12). These experiments show that the high-pressure CO₂ molding process is more reliable and effective than is a post-molding treatment with high pressure CO₂.

The CO₂ in-situ molded specimens were also denser than the air-molded samples, as seen in Tables 6 and 7. Because carbonation filled pores and cracks in the cement, the dry surface area should decrease upon significant carbonation, as seen in Tables 6 and 7, even when polymer fibers were present. The bulk density of the dry carbonated materials increased, as carbonates are generally denser than hydroxides—with one exception, sample 16, which was a H₂O₂-foamed sample (compared to non-foamed standard 2A). Similar results were found for cement slags. (See Table 7.) Note that the voidages for the carbonated samples decreased significantly as compared to samples set in air (Tables 6 and 7). These lower voidages demonstrate that the novel carbonated cementitious materials possess excellent barrier properties, e.g. to ionic transport. The decreased ionic permeabilities lend these cements to uses such as housing and marine applications. In addition, reinforcing polymer fibers blended with such cements would be less susceptible to degradation by reaction with ions transported in water, especially saltwater or wastewater.

TABLE 6 Porosity and Density Results, Fly Ash Samples BET Voidage (based on water Surface Bulk displacement: (sample Area, density, volume-water displaced)/ Sample m²/g kg/m³ sample volume 2A-fly ash, set in air 8.5 1.82 0.18  2B-fly ash, set with 5.4 1.94 0.065 CO₂ 5-fly ash, set with 8.4 1.95 0.049 CO₂ 15-fly ash, set in air, 5.7 0.049 then CO₂ 16-fly ash, set with 4.6 1.68 0.089 CO_(2,) foamed 17-fly ash, set with 7.0 1.80 0.015 CO_(2,) foamed

TABLE 7 Porosity and Density Results, Cement Slag Samples BET Voidage (based on water Surface Bulk displacement: (sample Area, density, volume-water displaced)/ Sample m²/g kg/m³ sample volume 4-set with CO₂ 3.8 2.00 0.14 8-set in air 5.8 1.69 0.29 9-set with CO₂ 0.4 1.93 0.14 10-set with CO₂, 4.7 1.93 0.15 foamed

This process can be conducted at any pressure above ˜400 psi, preferably between ˜600 psi and ˜2000 psi. Although there is no upper limit on pressure, as a practical matter it becomes increasingly more difficult to handle fluids above a pressure ˜5000 psi. A delivery pressure to the mold of ˜700-800 psi is particularly convenient in many applications, because this is the pressure at which carbon dioxide is delivered from a tank of liquid carbon dioxide at room temperature (i.e., this is the vapor pressure of carbon dioxide at room temperature). Subsequent molding would increase the pressure within the mold. The temperature should be between −56° C. (the triple point of CO₂) and 200° C., preferably between 0° and 50° C. More specifically, the pressure/temperature combination should be such as to produce a CO₂ density near or exceeding the critical density of CO₂, 0.46 g/cm³. For example, at 25° C. the density of CO₂ in a near-critical state of 1000 psi is 0.74 g/cm³. This density easily suffices to give uniformly carbonated products.

As used in the specification and claims, unless context clearly indicates otherwise, the term “carbon dioxide” refers to any liquid, gas, or supercritical fluid containing a substantial amount of CO₂, at least 20% by weight (as measured before reaction with, or dilution into, other components). The term “cement” or “cementitious material” refers to any calcareous material which, when mixed with appropriate amounts of water (and, optionally, other curing additives), can be used as a binder for aggregates formed from materials such as sand, gravel, crushed stone, organic polymers, and other materials. A cement may include such aggregate or polymeric materials as blended mixtures. Examples of cementitious materials include Portland cements, fly ash, and cement slags such as blast furnace slag.

The complete disclosures of all references cited in this specification are hereby incorporated by reference. In the event of an otherwise irreconcilable conflict, however, the present specification shall control. 

We claim:
 1. A cured cement at least 2 mm thick, wherein all interior portions of said cement that are at least 1 mm from the nearest surface of said cement comprise interlocking calcium carbonate crystals at least 10 μm in diameter.
 2. A cement as recited in claim 1, wherein the pH of said cement is below about
 8. 3. A cement as recited in claim 2, wherein said cement is reinforced with polymeric fibers that are stable at the pH of said cement.
 4. A cement as recited in claim 3, wherein said fibers comprise a polyamide, a polyolefin, a polyamide blend, or a polyolefin blend.
 5. A cement as recited in claim 3, wherein said fibers comprise a nylon.
 6. A cement as recited in claim 3, wherein said fibers comprise polypropylene. 